The invention relates to an optical coupling means for coupling light between two waveguide end surfaces which are arranged opposite one another.
Coupling means of the named type are used to couple light supplied in an optical waveguide into another waveguide. An example is a coupling means for coupling light supplied in a strip-type waveguide, e.g. an optical fiber, into a strip-type or layer-type waveguide that is integrated on a substrate.
For example, the integrated waveguide can be a coupling-in waveguide of an optical multi-channel filter. An example of such a filter is a filter according to the phased-array design, with a coupling-in surface in the form of an end surface of the layer-type integrated input waveguide of this filter, through which light enters into the input waveguide at a particular geometrical position, whereby the geometrical position influences an output wavelength of the optical filter.
Such optical filters according to the phased-array principle are used in particular as multiplexers or demultiplexers in optical wavelength multiplex operation (WDM), since they comprise a low insertion damping and a high cross-talk suppression.
An optical filter according to the phased-array principle comprises, as an essential component, several optical waveguides of differing optical length that are coupled to the layer-type coupling-in waveguide and that run in a curved fashion, with the waveguides forming a phase-shifter arrangement. The layer-type coupling-in waveguide distributes the light onto the waveguides of differing optical length.
In WO 96/00915 A1 (GR 94 P 1417 DE), it is specified that the center wavelength of a phased-array filter can be determined by the position of the supplying waveguide, which conducts the light into the coupling-in waveguide of the filter. In this way, the center wavelength of this filter can be adjusted precisely by means of the geometrical positioning of the end surface of the supplying strip-type waveguide in relation to the end surface of the coupling-in waveguide of the filter.
After the adjustment of the center wavelength, the end surface of the supplying waveguide and the end surface of the coupling-in waveguide of the filter are fixed relative to one another, e.g. in that the supplying waveguide is glued fixedly to the substrate of the filter.
Given a conventional manner of construction of such a filter, the center wavelength and channel spacing are determined by the layout of the filter and by the process technology.
With respect to many applications, there is a need for a filter that can be tuned or matched. With such a filter, it is purposely possible to select one of several channels, or the center wavelength of a filter comb can be matched to the requirements of operation, in order for example to compensate for the ageing of transmitting lasers.
Apart from classical monochromators, piezo-controlled Fabry-Perot resonators are predominantly offered on the market as tunable or matchable optical filters. These are manufactured in a micromechanical construction, and are for this reason expensive and not suitable for mass production and application.
Mach-Zehnder interferometers can be tuned (see N. Nakato et al.: xe2x80x9c128-Channel Polarisation-Intensive Frequency-Selection Switch using High-Silica Waveguides on Si,xe2x80x9d IEEE Photonics Technology Letters 2, p. 441 (1990)), but are periodic filters. For narrow-band applications, a cascading of several such filters, matched to one another, is thus necessary. The matching of a filter constructed in this way thereby becomes very expensive, because for this purpose a corresponding number of control currents must be regulated.
Phased-array filters can in principle be tuned by modifying the optical path length in the individual waveguides of the phase-shifter region of this filter, e.g. by means of thermo-optical and/or electro-optical effects (see EP 0 662 621 A1 (GR 94 P 1013 DE)).
The invention is based on the object of providing optical multichannel filters, easily tuned, with a strip-type or layer-type coupling-in waveguide comprising an end surface, in particular phased-array filters.
Advantageously, with the inventive coupling means it is possible to adapt a particular output wavelength of the filter, e.g. the center wavelength of a filter comb, to the requirements of the operation, in order for example to compensate for the modification of the transmitting lasers.
A particular advantage of the invention is that not only such tunable optical multi-channel filters, including multi-channel filters according to the spectrograph principle, but rather also optical intensity modulators and waveguide switches, can be realized.
The shifting means of the inventive coupling means can advantageously be realized in miniaturized form, i.e. with dimensions that are on the order of magnitude of the dimensions of the waveguide end surfaces.
The inventive coupling means advantageously comprises a compact construction, and thus has sufficient stability to ensure that the adjustment of a strip-type waveguide is maintained in relation to the position of another waveguide, in particular of a layer-type waveguide, and no worsening of the insertion damping thereby takes place.
The inventive shift means is preferably and advantageously constructed in such a way that, relative to a support point that is fixed relative to one of the two end surfaces, the expansion element expands and/or contracts in a direction parallel to the one end surface, and thereby moves the other end surface relative to the fixed support point. A fixed reference point is thereby given to which the expansion and/or contraction are oriented. The end surface that is moved relative to the fixed support point can be the end surface of a waveguide in which the light is supplied to the coupling means, and/or the end surface of a waveguide in which the light is conducted away from the coupling means.
The other end surface is advantageously mounted in a mounting element that can be moved parallel to the one end surface, which element is connected fixedly with the expansion element. It is thereby advantageous for the mounting element and the expansion element to be constructed in one piece, since in this way the assembly expense is reduced.
Preferably and advantageously, the mounting element is made of a ceramic material, and comprises a continuous opening in which a waveguide comprising the other end surface is housed and fixed.
In order to prevent the mounting element from being able to oscillate about the support point with the expansion element, it is advantageous to provide a guide means for the linear guiding of the mounting element parallel to the one end surface. Such a guide means preferably comprises two glide surfaces positioned opposite one another and arranged fixedly relative to an end surface, between which the mounting element is arranged and along which the mounting element can be moved parallel to the one end surface.
In a preferred and advantageous embodiment of the inventive coupling means, the displacement means can be controlled externally. For this purpose, this embodiment is advantageously constructed in such a way that the displacement means comprises a control means with which the expansion and/or contraction of the expansion element can be controlled.
The coupling means that can be controlled in this way is preferably fashioned in such a way that the expansion element is made of piezoelectric material and the control means produces an electrical field that acts on the piezoelectric material with a field strength that can be modified in a controllable fashion, and/or in such a way that the expansion element is made of material with a thermal expansion coefficient, and the control means comprises a means for the controlled heating and/or cooling of the expansion element.
In a further preferred embodiment of a controllable coupling means, the displacement means comprises a positioning motor that is fixed relative to the one end surface, and comprises an expansion element in the form of a final controlling element that is connected with the other end surface and that can be displaced by the positioning motor, and the control means controls the positioning motor.
In a preferred and advantageous construction of an inventive coupling means, the displacement means comprises a compensation means for the compensation of a position spacing between a relative actual position that the two end surfaces assume relative to one another and a relative target position that the two end surfaces are supposed to assume relative to one another.
This construction advantageously enables an automatic correction of the relative position that the two end surfaces assume relative to one another to a new relative position, the target position, when this is required by circumstances.
If a compensation means for compensation of the particular spacing between a relative actual position and a relative target position is provided, and the difference in position between the relative actual position and the relative target position is essentially proportional to a temperature difference prevailing in the environment of the coupling means, this coupling means can advantageously be constructed in such a way that the compensation means consists of an expansion element made of material with a thermal coefficient of expansion, and that the thermal coefficient of expansion and a dimension of the expansion element are selected parallel to an end surface relative to one another in such a way that a thermal expansion and/or contraction of the expansion element, caused by the temperature difference, essentially compensates the difference in position.
Such relations are for example present, as indicated already, in an optical filter according to the phased-array principle in relation to an output wavelength of this filter, whose position is temperature-dependent.
An advantageous compensation means that can compensate not only temperature-caused positional differences but also positional differences caused in other ways, e.g. caused by wavelength modifications, is fashioned in such a way that it comprises an expansion coefficient made of piezoelectric material, a means for determining the positional spacing between the relative actual position and the relative target position, and a control means for producing an electrical field strength that acts on the piezoelectric material and that is proportional to the determined positional difference in such a way that the piezoelectrical expansion and/or contraction of the expansion element produced by this field strength essentially compensates the positional difference.
Another advantageous compensation means is constructed in such a way that the compensation means comprises a positioning motor that is fixed relative to the one end surface and an expansion element in the form of a final positioning element that is connected with the other end surface and can be moved by the positioning motor, a means for determining the positional difference between the relative actual position and the relative target position, and a control means for controlling the positioning motor dependent on the determined positional difference in such a way that the positioning motor essentially compensates the positional difference. This embodiment can also compensate not only temperature-caused positional differences but also positional differences caused in other ways, e.g. caused by wavelength modifications. In a particularly preferred embodiment of an inventive coupling means, one of the two end surfaces is an end surface of a waveguide of an optical wavelength filter that serves for the coupling of light into or out of the filter, whereby a tunable multichannel filter is realized.
In a concrete embodiment of such a filter, the wavelength filter consists of a filter according to the phased array principle, in which the position of the center wavelength or of other output wavelengths is temperature-dependent.
For example, in a phased-array filter in the InGaAsP material system, the temperature dependence of the transmission curves of approx. 0.01 nm/K in the SiO2 material system and approx. 0.1 nm/K has up to now been an obstacle to a successful use of the system. In use, temperature differences of more than 100 K can occur in these optical filters, whereby problems occur given channel spacings of only a few nanometers.
With the coupling means, an optical fiber according to the phased-array principle can be realized that comprises a coupling-in surface into which light enters at a particular position, whereby the geometrical position influences an output wavelength or, respectively, center wavelength of the filter, in which the temperature sensitivity of the filter can be compensated in a particularly reliable and simple manner.
Up to now, an active temperature regulation has been carried out for the solution of this problem, and a uniform temperature has been produced in the filter region. For this purpose, however, an additional specification and monitoring expense is required.
In principle, it is also possible to manufacture filters from glasses having a low temperature coefficient. However, these require a considerable development expense.
On the basis of the coupling means, in of the invention, a means for coupling light in, which means can be subsequently moved parallel to the coupling-in surface of the filter, and a temperature-sensitive element are present. The movable means is connected with the temperature-sensitive element in such a way that the geometrical position on the coupling-in surface at which the light enters into the optical filter can be moved dependent on the temperature.
In this way, in the phased-array filter a reliable temperature compensation can be produced by means of a simple mechanical displacement of a position for coupling light in, without requiring an expensive cooling apparatus or heating means.
In the coupling means of the invention, the temperature-sensitive element and the displaceable means advantageously consist essentially only of the compensator, whose displacement is due to a thermal length expansion. By this means, both functions are perceived by only one element, which in addition operates passively and requires no monitoring at all.
It is thereby advantageous to design the dimension, e.g. length, and material of the compensator dependent on the temperature coefficient of the optical filter. This is possible in principle because both the temperature coefficient of a phased-array filter and also the dispersion exhibit linear dependencies. The likewise linear expansion of the compensator can thus be adapted precisely to the optical filter by the selection of the material and/or of the dimension of the compensator in the direction of the expansion.
The compensator is advantageously fastened mechanically to the optical filter so that a fixed reference point is given to which the displacement can be oriented. The displaceable means for coupling light in preferably comprises a coupling-in fiber with a mount. The compensator and the mount for the coupling-in fiber are preferably fashioned in one piece from a suitable material, since by this means the assembly expense is reduced.
In order to prevent the coupling-in fiber from oscillating about the fastening point with the compensator, it is advantageous to provide a mechanical guide for the compensator and the mount for the coupling-in fiber. Such a mechanical guide preferably comprises a covering plate and one or two supports that are made of materials with adapted expansion coefficients. The compensator and the mount fastened thereto with the coupling-in fiber are then displaced in a defined fashion in one direction, guided by the cover plate and a base plate.
On the basis of the coupling means according to the invention, another preferred embodiment of the filter is realized in which the temperature-sensitive element comprises a temperature sensor with an electrical output and the displaceable means comprises an electrically controllable stepped motor. Likewise, the displaceable means can comprise a piezocrystal. By means of such an arrangement, dependent on the temperature a coupling-in fiber can likewise be displaced in defined fashion on the coupling-in surface, so that the temperature dependence of the optical filter is compensated.
The coupling means according to the invention is not limited to filters according to the phased-array principle. The optical filter in which the inventive coupling means can be applied can be for example an integrated optical multi-channel filter according to the spectrograph principle.
In such a filter, the light conducted in a layer waveguide is bent at a reflection grid inserted into the waveguide, and is simultaneously imaged from a coupling-in point to a coupling-out point whose position is wavelength-dependent. If the coupling-in point is displaced in relation to a coupling-in end surface, the position of the coupling-out points wanders corresponding to the optical imaging, so that here as well, similar to the filter according to the phased-array principle, a fine tuning of the wavelength channels is possible via the position.
If the integrated optical filter according to the spectrograph principle is for example designed according to what is called the flat field principle, coupling-out waveguides can also be moved in principle for the tuning. In this case, an inventive coupling means is to be arranged opposite a coupling-out end surface of the waveguide, from which light is coupled out from the filter.
The possibility of tuning grid spectrographs by the displacement of coupling-in and coupling-out waveguides was published in K. A. McGreer: xe2x80x9cTunable Planar Concave Grating Demultiplexerxe2x80x9d, IEEE Photonics Technology Letters 8, page 551 (1996), but this possibility is based on a different principle than the inventive coupling means.
The inventive coupling means is not limited to optical filters, but rather can in principle be applied in all optical means in which waveguide end surfaces of two or more optical waveguides are positioned opposite one another for the coupling-in and/or coupling-out of light between these waveguides, and in which a relative displacement between the end surfaces parallel to an end surface is desired or necessary.
A preferred embodiment of such a coupling means is fashioned in such a way that the end surfaces positioned opposite one another are strip-type optical waveguides comprising essentially the same cross-section. The strip-type waveguides can be optical fibers and/or integrated strip-type waveguides whose cross-section is not elongated, in distinction from layer waveguides, but rather comprises essentially equal height and width.
With such a coupling means, in connection with a control means according to the invention, an adjustable optical attenuator can be realized in which the coupling attenuation between two strip-type waveguides whose end surfaces are positioned opposite one another increases in a controlled fashion with an increasing lateral displacement of the two end surfaces relative to one another.
With such a coupling means, a waveguide switch can also advantageously be realized whose manner of operation is analogous to that of the attenuator. A preferred embodiment for this purpose is fashioned in such a way that end surfaces, arranged next to one another, of two or more other strip-type waveguides are arranged opposite the end surface of a strip-type waveguide, and the displacement means is fashioned in such a way that the end surface of the one strip-type waveguide and the end surfaces of the other strip-type waveguides can be displaced relative to one another by at least a distance between adjacent end surfaces of the other strip-type waveguides. By means of this measure, it is selectively possible to bring the end surface of the one strip-type waveguide into a position opposite the end surface of another strip-type waveguide or the end surface of another different strip-type waveguide. The displacement means need only be dimensioned in such a way that in the relative displacement of the end surfaces at least the spacing of two adjacent end surfaces of the other strip-type waveguides can be bridged over.